Condensate drains, which automatically drain condensate forming in steam lines or pressure lines into a condensate collecting vessel without gas, for example in the form of water vapour or compressed air, exiting to a notable extent from the line, usually comprise fill level monitoring.
In compressed air technology, condensate drains are used to drain condensates out of pressure line networks. The condensates essentially arise from the humidity of the ambient air which is sucked in by the compressed air compressor. There are various designs of condensate drains. In principle, they operate in such a way that a valve is opened, as a result of which the condensate is expelled from the compressed air network by the pressure present therein. The devices differ according to the type of control of the valve and its power supply.
Float-type drains operate with a hollow body, which is raised by its buoyancy in the collecting condensate and thus actuates a valve. This valve usually opens a drain opening indirectly by means of a servo-control, through which drain opening the condensate is ejected.
Furthermore, conventional electrically operated, time-controlled solenoid valves open at adjustable, but preset time intervals. A drawback is that these solenoid valves also open with an absence of condensate independently of the fill level, as a result of which high energy losses arise due to discharged compressed air.
Electronically level-controlled valves, on the other hand, detect an accumulating quantity of liquid via an electronic sensor, which enables quantitative monitoring of the fill level. When a specific quantity is reached, a valve is opened and precisely this quantity is drained off without losses of compressed air. Condensate drains with electronically level-controlled valves are therefore particularly advantageous on account of the small compressed air and therefore energy losses and have become established.
Particular importance is attached to condensate drains which operate with a capacitive measurement for the fill level detection and in particular with servo-controlled diaphragm valves. The capacitive measurement detects the fill level of a condensate collecting vessel on the basis of the electrical capacitance, which changes with the fill level when the condensate flows in as a dielectric medium. Since the condensate is polluted with oil and/or dirt to a varying extent depending on the operating and ambient conditions and these additional contents have a considerable influence on the capacitance, this fill level monitoring may possibly not be able to be evaluated unequivocally in respect of the actual fill level.
EP 0391250 B1 describes, for example, a device for draining condensate from compressed air systems or suchlike, with a collecting chamber for condensate permanently connected to the compressed air system and with a diaphragm valve closing the outlet of the collecting chamber, wherein two capacitive sensors detecting different fill levels and controlling the diaphragm valve via electronics and a control valve are disposed in the collecting chamber. The sensors are disposed at a vertical distance from one another in a pipe projecting vertically into the collecting chamber and closed at the external end, said sensors being connected electrically to a control valve controlling a pilot-control outlet from the collecting chamber.
Therefore, a need exists for a method for detecting the actual fill level more reliably and a circuit for fill level monitoring or fill level control.